1. Field of the Invention
The present invention relates to an eddy-current sensor for nondestructive testing used for detecting object's shapes, defects and so on nondestructively.
2. Description of the Related Art
Eddy-current testing (ECT) technique is frequently utilized for nondestructive testing of important metal machine parts used in a nuclear power plant, an aircraft, and so on. Generally, such an ECT probe for nondestructive testing using the eddy-current includes mainly an exciting coil and a detector coil for detecting a magnetic field based on an eddy-current induced by an alternating magnetic field generated by the excited coil. Such a technique is described in for example, Japanese Patent Publications Nos. 07-083884A, 09-189682A, 11-248685A and 2002-090490A.
Further, some of the inventors propose an ECT probe for inspecting printed circuit boards, including a meander-type exciting coil and a figure-of-eight-type pick-up coil for the eddy-current detection, described in for example, T. Miyagoshi, D. Kacprzak, S. Yamada and M. Iwahara, “Feasibility of Inspecting Defects in Printed Circuit Boards by Using Eddy-Current Testing Techniques”, Journal of the Magnetics Society of Japan, Vol.23, No.4–2, pp. 1613–1616, 1999, and S. Yamada and M. Iwahara, “Trend of Detection Techniques Using Planar-Type Micro-Eddy-Current Testing Probes”, Journal of the Magnetics Society of Japan, Vol.23, No.7, pp.1817–1825, 1999.
Because the above-mentioned conventional ECT probe uses a coil as a means of detecting the magnetic field based on the eddy-current, it has a limit of miniaturization and sensitivity improvement. That is, the detection coil has a certain amount of length, width and thickness. Therefore, the conventional ECT probe has been practically able to detect only relatively large defects and changes in the order of millimeter or more.
Recently, the ECT probe, however, has been largely required to be used for very fine testing such as an inspection of micro-defects on an object's surface and an inspection of micropatterns of a printed circuit board. The following problems occur when the ECT probe with the conventional structure is used for the purpose to meet such a requirement:
(a) The conventional ECT probe has a physical limit for reducing the number of turns in the detection coil, a diameter of the coil and a coil length in order to improve its resolution;
(b) The probe's sensitivity decreases by reducing the number of turns in the coil, a diameter of the coil and a coil length in order to improve its resolution, because coil's sensitivity is proportional to a coil's cross-section and the number of turns in the coil;
(c) The sensitivity decrease damages reliability of signals themselves because the decrease lowers the signal-to-noise ratio.
Especially, when the detection coil is set on the opposite side to a subject in relation to the exciting coil in order to prevent a protrusion from being formed on the near side to the subject in relation to the exciting coil, no high-reliability test is expected to be performed without the detection coil more sensitive.